The present invention relates to an ultra-high vacuum apparatus, and more particularly to an ultra-high vacuum apparatus suited for use in a field emission type scanning electron microscope.
In a scanning electron microscope, a field emission type electron gun is generally used rather than a thermionic emission type electron gun in order to obtain an electron beam with which a sample or specimen is irradiated. This is because an electron current density in the field emission is remarkably higher than that in the thermionic emission and the diameter of an electron beam source is extremely smaller in the field emission than in the thermionic emission. However, an ultra-high vacuum is required for obtaining a stable field emission current. Though a degree of vacuum of 10.sup.-7 Torr and less is generally called an ultra-high vacuum, an ultra-high vacuum of 10.sup.-9 Torr and less, and typically 10.sup.-9 Torr to 10.sup.-10 Torr, is indispensable for obtaining a stable field emission current.
According to a field emission type scanning electron microscope, an electron gun chamber in which a field emission gun is arranged is evacuated by a vacuum pump such as a rotary pump through a specimen chamber in which a specimen is disposed and then is continued to be evacuated by a vacuum pump such as an ion pump. The electron gun chamber is maintained at an ultra-high vacuum of the order of 10.sup.-9 Torr to 10.sup.-10 Torr, while the specimen chamber is kept at 10.sup.-8 Torr and more, and typically 10.sup.-5 Torr to 10.sup.-6 Torr.
When the specimen is exchanged, the specimen chamber is made open to the atmosphere to replace the old specimen by a new one. In this case, it is desirable to isolate the electron gun chamber from the specimen chamber in vacuum continuum to prevent the electron gun chamber from being vacuum-broken. Therefore, a valve means is provided for connecting and disconnecting the electron gun chamber with and from the specimen chamber each time the specimen is exchanged. The valve means comprises a vacuum-tight seal, an opening for communicating the electron gun chamber and the specimen chamber with each other, and a manipulator for bringing the vacuum-tight seal into close contact with an end surface of the opening so as to close the same and for releasing the close contact between the vacuum-tight seal and the opening end surface so as to open the opening. Walls of the electron gun chamber and the specimen chamber are made of a metal such as stainless steel, including a member forming the opening. The vacuum-tight seal, however, is generally made of a material other than a metal to maintain the close contact between the vacuum-tight seal and the opening end surface even where the opening is closed and opened repeatedly.
On the other hand, in advance of assembling an ultra-high vacuum apparatus, components thereof are individually baked, including the vacuum-tight seal. The apparatus is assembled with the baked components and then further baked in advance of formal specimen observations. The baking treatment is normally carried out at a high temperature of about 250.degree. C. to 400.degree. C. for outgassing, i.e. degassing the surface of the apparatus exposed to the vacuum atmosphere therein. Thus, the vacuum-tight seal should be made of a material resisting such a high temperature.
In view of the foregoing, the vacuum-tight seal is generally made of polyimide or fluorocarbon polymer. Examples employing these materials for a vacuum-tight seal are disclosed in Japanese laid-open Patent Document Nos. 153970/1991 and 234973/1991.
However, the outgassing of polyimide or fluorocarbon polymer is larger than the outgassing of other structural materials of the ultra-high vacuum apparatus such as metallic materials like stainless steel. Comparing outgassing rate (Torr.multidot.l/sec/cm.sup.2 at 200.degree. C.) under an identical condition, it is known that the outgassing rate of polyimide is approximately 1000 times as large as that of stainless steel. Therefore, employing polyimide for a vacuum-tight seal increases the baking time and the ultimate pressure.